Magical Boy Gender is a gender that feels connected to Magical Boys/Mahou Shounen. It's a rather simple Xenogender that's easily explained. Magical Boys like Tuxedo Mask, Battle Lover Scarlet, etc. were in mind when this was made, but "Magical Boy" can mean alot of things, which means many many people who identify as this gender can experience it in many many different ways.
It was coined by MxBeyzarre on twitter, he designed the flag as well.
Resources
https://twitter.com/MxBeyzarre/status/1302296820828250117